This section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in filenames.

To determine within a startup script whether or not Bash is running interactively, test the value of the ‘-’ special parameter. It contains `i` when the shell is interactive. For example:

```
case "$-" in
*i*)	echo This shell is interactive ;;
*)	echo This shell is not interactive ;;
esac
```

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:

```
if [ -z "$PS1" ]; then
        echo This shell is not interactive
else
        echo This shell is interactive
fi
```

## Invoked as an interactive login shell, or with --login
When Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file `/etc/profile`, if that file exists. After reading that file, it looks for `~/.bash_profile, ~/.bash_login, and ~/.profile`, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, Bash reads and executes commands from the file `~/.bash_logout`, if it exists.

## Invoked as an interactive non-login shell
When an interactive shell that is not a login shell is started, Bash reads and executes commands from `~/.bashrc`, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force Bash to read and execute commands from file instead of `~/.bashrc`.

So, typically, your ~/.bash_profile contains the line
```
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
```
after (or before) any login-specific initializations.

## Invoked non-interactively
When Bash is started non-interactively, to run a shell script, for example, it looks for the variable `BASH_ENV` in the environment, expands its value if it appears there, and uses the expanded value as *the name of a file to read and execute*. Bash behaves as if the following command were executed:
```
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
```
but the value of the `PATH` variable is not used to search for the filename.

As noted above, if a non-interactive shell is invoked with the --login option, Bash attempts to read and execute commands from the login shell startup files.